DOVER, N.H. — A martial arts instructor active in community theater was charged Saturday in the death of a 19-year-old University of New Hampshire student who disappeared days earlier.

Elizabeth “Lizzi” Marriott, a marine biology major who volunteered at the New England Aquarium, is believed to be dead and law enforcement officials continued to search for her body, Assistant Attorney General James Vara said Saturday.

Seth Mazzaglia, 29, of Dover, who’s charged with second-degree murder, knew Marriott, Yara said, but he declined to say how or what led to Mazzaglia’s arrest. He said police had “credible” information leading them to believe Marriott is dead, though he wouldn’t elaborate.

A family friend, Dawn Downey of Westborough, Mass., described Marriott as an attractive young woman who was active in chorus in high school and was prom queen during her junior year. Marriott was helping to put herself through college by working at Target.

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“She was just a good girl. That’s probably what got her in trouble. She was too trusting and she was beautiful. Those two things will kill you,” Downey said.

Marriott had attended a class Tuesday night and made plans to visit friends in Dover. Her cellphone was last used in Dover that night, according to fliers that family members posted around town. Her car was later found in a parking lot on campus in Durham.

“We were greatly saddened to learn of Lizzi’s death and we extend our deepest sympathies to her family and friends,” UNH President Mark W. Huddleston said in a statement. “Lizzi was a new member of the university community and will be missed in our classrooms. Our focus now will be on supporting our campus community during this difficult time.”

The university was making counselors available to students. A moment of silence was planned in Marriott’s honor at the school’s hockey game Saturday night.

Marriott was a 2011 graduate of Westborough High School in Massachusetts. She lived with an aunt in Chester and commuted to the Durham campus. A message seeking comment was left with her parents, Robert and Melissa Marriott of Westborough.

Downey’s daughter Nicole, who was Marriott’s best friend, described Marriott as a “fun-loving and goofy” young woman who got along easily with others and had a wide circle of friends. She also was a good student who loved animals and science and volunteered at the New England Aquarium in Boston, Nicole Downey said.

“It was a shock to me when I found out she was missing. That was not like her at all,” she said. “She was just all around a great person. I don’t know why anyone would want to hurt her.”

Mazzaglia, who graduated from the University of New Hampshire in 2006 with a bachelor’s degree in theater, was active in the arts, performing in several dozen plays and musicals over the last decade in southern New Hampshire.

He also was a martial arts instructor who described himself on websites as having a black belt in karate and being familiar with several forms of martial arts and weapons. And he described other interests including juggling.

Robert Modee, a friend of Mazzaglia’s and a martial arts instructor in Kittery, Maine, said he was struggling to comprehend how his friend could be charged with murder.

“I don’t understand what’s going on right now,” he told the Foster’s Daily Democrat in Dover, N.H. “I’m completely floored with all this.”

Mazzaglia is scheduled to be arraigned Monday. A working telephone number for him could not immediately be located. It was unclear whether he had a lawyer yet.

CORRECTION:

An earlier version of this story contained an inaccurate headline. As of Saturday afternoon, Oct. 13, Assistant Attorney General James Vara said Elizabeth “Lizzi” Marriott is believed to be dead but law enforcement officials continued to search for her body. Her body has not been recovered.